The type of transformer that I am concerned with is typified by the transformer disclosed in U.S. Pat. No. 4,734,975--Ballard and Klappert, assigned to the assignee of the present invention, which patent is incorporated by reference in the present application. This transformer comprises a core made up from a plurality of thin strips of amorphous steel and preformed coil subassemblies respectively surrounding the legs of the core. Each of the preformed coil subassemblies in such a transformer is typically made by winding insulation-covered wire about a hollow form of electrical insulating material. The internal surface of this hollow form defines a generally rectangular coil window that surrounds the associated leg of the core. This window is bounded by two spaced-apart generally parallel end walls and two spaced-apart sidewalls.
The above-described coil window is not exactly rectangular in cross-section. Typically, the internal surfaces of its sidewalls are slightly concave and its corners are rounded or have a bevel (for reasons associated with the manufacture of the preformed coil assembly). These departures from an exact rectangular form make it difficult to make highly efficient use of the window cross-section for receiving the core leg, particularly if the core leg has a rectangular cross-sectional periphery intended to closely fit the coil window. The concave internal surfaces of the sidewalls create wasted space between the coil window and such a core leg, and the rounded or beveled corners tend to interfere with ideal positioning of the corners of the core leg.